While cathodoluminescence (CL) detectors are generally known, in CL detectors using a solid-state detector as the detection element, the photodiodes are positioned inside the electron microscope vacuum chamber close to the beam impact point. A glass cover is normally placed in front of the solid-state detector to protect it from energetic backscattered electrons that can also contribute to the detector signal. However, the glass cover in front of the solid-state detector is actually a powerful CL emitter and acts as an extremely efficient converter of backscattered electrons to light which would be detected by the solid-state photodiode. The total signal detected bears little or no relationship to the CL signal generated by the specimen under observation.
A high efficiency detector providing discrimination against backscattered electrons is disclosed in U.S. Pat. No. 5,264,704 being issued to J. C. H. Phang et al, the present inventors, on Nov. 23, 1993. In this patent, a parabolic light reflection device is combined with a photo-sensitive solid state detection device which can easily be mounted and removed from the vacuum chamber specimen stage of an electron microscope. The detector additionally includes a generally flat photosensitive element mounted on a flat supporting plate in the same plane as the specimens under investigation so that backscattered electrons from the specimen will not strike the photosensitive area of the photosensitive element and will be effectively absorbed by the parabolic light reflector.
Although the detector apparatus described in U.S. Pat. No. 5,264,704 overcomes some of the limitations existing in the prior art, when using gallium arsenic semiconductor as a specimen, as much as 6% of the electrons from the primary beam can still reach the photo-sensitive detection element. This has been found to be undesirable for quantitative cathodoluminescence measurements.